The present disclosure relates to the frequency responses measurement in devices, where the input and output signals are in different frequency ranges, and, in particular, to the phase frequency distortions measurement in precise measuring devices used in the wireless communication systems.
Wireless networking systems have become a prevalent means in the communication industry. In such systems it is very important to measure/analyze with a high degree of accuracy the various properties of a transmitted/received modulation signal. Therefore a large and perpetually increasing demand exists for high precision spectrum analyzers, vector/signal analyzers etc operating in the RF range.
A typical block diagram of an analyzer is shown in FIG. 1. The down converter 100 transfers the part of the input signal spectrum to be analyzed to the operational frequency range of the analog to digital converter (ADC) 101. The ADC 101 transforms incoming continuous signal into a sequence of digital samples. A processor 102 carries out the necessary analysis of the properties of the processed signal with the presentation of the received results at the display 103.
It is important for the down converter not to create spurious responses, which may substantially distort the processed signal. To attain such a purpose, a conventional down converter contains usually several conversion stages (three or four) with an appropriate selection of the local oscillators frequencies. At each conversion stage a filter is used to separate out the desired frequency components. These filters inevitably introduce frequency distortions in the processed signal. Typically, the area of application of an analyzer requires knowledge of the signal distortions with a high degree of accuracy. For this reason, it is necessary to measure the down converter frequency response and, in particular, the phase frequency distortions.
The down converter 100 and the ADC 101 form the device under test (DUT) 104 that is the subject of the measurement according to the present invention. A feature of the DUT 104 is that its input and output signals lie in different frequency ranges.
The known methods of frequency responses measurement are based on a comparison of the output signal of the device under test with the input signal or with a duplicate of the input signal. However, when the input and the output frequency ranges are different, it is impossible to compare the phases of sine wave components in the input signal and output signal directly: the difference between the phases varies in time continuously.
The present invention describes a method and apparatus for measurement of the phase distortions in a device with different input and output frequency ranges.